Excessive retinal vascular permeability contributes to the pathogenesis of diabetic macular edema (DME), a leading cause of vision loss in working-age adults. We recently reported that activation of the kallikrein-kinin system (KKS) by intravitreal injection of carbonic anhydrase 1 (CA-I) in rats resulted in a rapid increase in retinal vascular permeability (RVP) followed by the development of focal areas of increased vascular leakage to fluorecein-labeled 2x106 Dal dextran conjugate at 48 hrs post injection (Gao et al. Nature Medicine 2007, Appendix 1). In addition, we showed that in the presence of diabetes, CA-I caused intraretinal thickening, measured by 3D optical coherence tomography (OCT), at this time point. To our knowledge, this is the first report of retinal thickening in a diabetic rodent that is similar to clinically evident retinal edema. We reported that CA-I-induced retinal edema was decreased by complement 1 inhibitor, neutralizing antibody to prekallikrein, and bradykinin receptor antagonism. We demonstrated that plasma kallikrein, factor XII, and high molecular weight kininogen, are present in their activated forms in vitreous from people with diabetic retinopathy. In preliminary studies, we have shown that intravitreal injection with activated purified plasma kallikrein, kallikrein(act), induced both acute diffuse RVP at 30 min and focal areas of RVP and retinal leukostasis at 48 h post injection. In addition, we show that the effects of kallikrein(act) on the development of focal areas of leakage to fluorecein-labeled 2x106 Dal dextran are increased in both rats and mice with diabetes compared with nondiabetic controls. Using cultured retinal microvessel endothelial cells and astrocytes, we have identified both bradykinin-dependent and -independent mechanisms of kallikrein action. Although the KKS has been identified as a key pathway of vascular inflammation and vasogenic edema in stroke and angioedema, little is known regarding the actions of the KKS on the retina and its potential role in diabetic retinopathy. This grant will investigate these effects of the KKS on retinal vascular function and examine the contribution of this system to diabetic retinal edema. Our studies will examine the hypothesis that diabetes increases the actions of kallikrein on the retina and that increased kallikrein action leads to increased retinal vascular permeability and edema via a combination of B2-R activation and kallikrein's direct effect on extracellular proteolysis. We will investigate the contributions of both bradykinin receptor-dependent and -independent mechanisms of plasma kallikrein action on the retina using both in vivo studies on diabetic and nondiabetic rodents and in vitro studies using retinal microvessel endothelial cells and astrocytes. In collaboration with Dr. James Fujimoto (Massachusetts Institute of Technology), we will characterize the effects of the KKS and diabetes on retinal ultrastructure using OCT. This grant will provide critical new information on plasma kallikrein actions in the retina that contribute to both the diffuse and focal lesions of RVP, inflammation, and retinal thickening, which have been implicated as primary causal factors and characteristics of DME. PUBLIC HEALTH RELEVANCE: This grant will characterize the role of plasma kallikrein-induced inflammation in diabetic retinopathy. This grant is based on exciting new data that has identified a hormone system that triggers retinal edema in diabetic rodent models. These studies could reveal new therapeutic strategies to treat diabetic macular edema, a leading cause of vision loss.